The symbiosis between the gram-negative soil bacteria, Rhizobiaceae and Bradyrhizobiaceae, and legumes such as soybean, is well documented. The biochemical basis for these relationships includes an exchange of molecular signaling, wherein the plant-to-bacteria signal compounds include flavones, isoflavones and flavanones, and the bacteria-to-plant signal compounds, which include the end products of the expression of the bradyrhizobial and rhizobial nod genes, known as lipo-chitooligosaccharides (LCOs). The symbiosis between these bacteria and the legumes enables the legume to fix atmospheric nitrogen for plant growth, thus obviating a need for nitrogen fertilizers. Since nitrogen fertilizers can significantly increase the cost of crops and are associated with a number of polluting effects, the agricultural industry continues its efforts to exploit this biological relationship and develop new agents and methods for improving plant yield without increasing the use of nitrogen-based fertilizers.
U.S. Pat. No. 6,979,664 teaches a method for enhancing seed germination or seedling emergence of a plant crop, comprising the steps of providing a composition that comprises an effective amount of at least one lipo-chitooligosaccharide and an agriculturally suitable carrier and applying the composition in the immediate vicinity of a seed or seedling in an effective amount for enhancing seed germination of seedling emergence in comparison to an untreated seed or seedling.
Further development on this concept is taught in WO 2005/062899, directed to combinations of at least one plant inducer, namely an LCO, in combination with a fungicide, insecticide, or combination thereof, to enhance a plant characteristic such as plant stand, growth, vigor and/or yield. The compositions and methods are taught to be applicable to both legumes and non-legumes, and may be used to treat a seed (just prior to planting), seedling, root or plant.
Similarly, WO 2008/085958 teaches compositions for enhancing plant growth and crop yield in both legumes and non-legumes, and which contain LCOs in combination with another active agent such as a chitin or chitosan, a flavonoid compound, or an herbicide, and which can be applied to seeds and/or plants concomitantly or sequentially. As in the case of the '899 Publication, the '958 Publication teaches treatment of seeds just prior to planting.
More recently, Halford, “Smoke Signals,” in Chem. Eng. News (Apr. 12, 2010), at pages 37-38, reports that karrikins or butenolides which are contained in smoke act as growth stimulants and spur seed germination after a forest fire, and can invigorate seeds such as corn, tomatoes, lettuce and onions that had been stored. These molecules are the subject of U.S. Pat. No. 7,576,213.
There is, however, still a need for systems for improving or enhancing plant growth.